Hard to explain, please help

Because you have sorted your data by column B here is an alternative to SUMPRODUCT()

=COUNTIF(INDEX($A$2:$A$8761,MATCH(1,$B$2:$B$8761,0)):INDEX($A$2:$A$8761,MATCH(1,$B$2:$B$8761,0)+COUNTIF($B$2:$B$8761,1)-1),">=600")

Change the 1's in the formula to cell ref holding the hour ending value so that by changing the value in that cell the formula will recalc for the new value